Melanoma is a cancer with high mortality and is projected to cause almost eight thousand deaths during 2008 in the US alone. Though the carcinogenic mechanisms that lead to melanoma remain poorly understood, melanoma is thought to originate from a rare melanocyte stem cell that resides in the skin. Recently, we have found that the transcription Pax3 is expressed by these uncommon melanocyte stem cells. Although Pax3 is required for melanocyte development during embryogenesis, it not found in normal mature melanocytes. Its presence in melanocyte precursors therefore suggests that Pax3 promotes a melanocytic commitment but blocks terminal melanocyte differentiation. If this proposed schema is true, then in normal melanocyte precursors, maintenance of Pax3 expression preserves pluripotency, and its repression induces differentiation. If Pax3 plays these roles in melanocytes, then persistent Pax3 expression might result in the uncontrolled cell growth and loss of terminal differentiation in melanomas. Indeed, we have shown that Pax3 is expressed in melanoma cell lines and primary tumors. The major objective of this application is to elucidate the mechanisms by which Pax3 regulates melanocyte stem cell maturation and how this protein becomes subverted in conversion to a malignant state. The long range goals of our studies are to determine the molecular biology of the melanocyte stem cell in mature skin and how these pathways are disrupted in melanoma. The hypothesis guiding this proposal is that Pax3 regulates a critical control point in the normal maintenance of melanocyte stem cells and this control point is dysegulated in melanoma cells. In Aim I we exploit the cell restricted expression pattern of Pax3 in the melanocyte stem cell to create mouse models for isolation and study of this rare cell population. We also target Pax3 in these cells to test the necessity of Pax3 in melanocyte precursors. In Aim II, we investigate the role of Pax3 in melanomas to determine whether and how this protein contributes to the tumor phenotype. Finally in Aim III we explore the role of the canonical Wnt signaling pathway in the melanocyte stem cell and how members of this pathway affect Pax3 function.